The present invention relates to a dryer section in a paper machine, comprising so-called normal dryer groups provided with a single-wire draw and in which drying cylinders are arranged in an upper row and reversing cylinders or rolls are arranged in a lower row. The dryer section has at least one so-called inverted group, in which the drying cylinders and the reversing cylinders or rolls are arranged in rows in an inverted order with respect to the order of arrangement in the normal dryer groups. The present invention also relates to a method for improving the drying characteristics of a paper web in the dryer section.
In the prior art, in multi-cylinder dryers in paper machines, twin-wire draw and/or single-wire draw is/are employed. In a twin-wire draw, the groups of drying cylinders include two wires which press the web against heated cylinder faces, one wire from above and the other one from below. Between the rows of cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible of fluttering, possibly causing web breaks. For this reason, in recent years, increasing use has been made of the single-wire draw, in which there is just one drying wire in each group of drying cylinders. The web runs on support of the drying wire through the whole group so that, on the drying cylinders, the drying wire presses the web against the heated cylinder faces, and on the reversing cylinders arranged between the drying cylinders, the web remains at the side of the outside curve. Thus, in a single-wire draw, the drying cylinders are arranged outside the wire loop, and the reversing cylinders are arranged inside the loop of the drying wire.
In prior art normal dryer groups with a single-wire draw, the heated drying cylinders are placed in the upper row, and the reversing cylinders are placed in the lower row. The upper and lower rows are usually horizontal and parallel to one another. In the assignee's Finnish Patent No. 54,627 (corresponding to U.S. Pat. No. 4,202,113, the specification of which is hereby incorporated by reference herein), an arrangement is described in which normal single-wire groups, such as those described above, and so-called inverted single-wire groups are arranged one after the other. In the inverted groups, the heated drying cylinders are placed in the lower row and the reversing suction cylinders or rolls in the upper row. As a result of this arrangement, the web is dried substantially symmetrically from both of its sides.
In the prior art, reference is also made to International Patent Applications WO 88/06204 and WO 88/06205 of Messrs. Beloit Corp. which describe a dryer section consisting of normal and inverted cylinder groups.
In the following description, the terms "normal (dryer) group" and "inverted (dryer) group" are used and generally connote the cylinder groups with a single-wire draw similar to those described above.
In the group gap draws between inverted and normal drying groups, various problems have occurred, for which the present invention offers novel efficient solutions. These problems include the large length of the dryer section, which increases the cost of the dryer section and of the machine hall. Problems have also occurred in the runnability of the dryer section and in the web transfer, problems arising from differences in speed between different wires, problems in the removal of broke, and problems related to transverse shrinkage of the web. Generally, these problems tend to become worse with increasing running speeds of the paper machines.